rockchip: back-to-bootrom: add 'back-to-bootrom' support for AArch64

The back-to-bootrom support for Rockchip is equivalent to an
(assembly) implementation of setjmp/longjmp (i.e. it saves the
stack-pointer, link-register and callee-saved registers). Up until
now, this had only been implemented for AArch32 (i.e. ARMv7 or older),
which puts the new ARMv8 devices (which boot in AArch64 mode) at a
slight disadvantage.

To allow use of the 'back-to-bootrom' feature on new devices (e.g. the
RK3368), this commit adds an implementation for AArch64.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>